As described in U.S. Pat. No. 6,153,094, the filter medium, when arranged as a random heap of five-centimeter cubes of soft highly-absorbent foam, has the advantageous property of holding the wastewater physically “up in the air”. That is to say: the wastewater, having being dosed onto the topmost portions of the filter medium, does not then drain straight down through the whole filter medium, but rather the wastewater is absorbed in, and retained in, the cubes of water-absorbing foam material when the dosing episode ends.
The water remains held up in, and by, the absorbent foam material of the cubes, during the period between dosings. The water proceeds gradually and progressively step-by-step down through the heap or stack, as subsequent dosings are administered to the stack, whereby large quantities of water are held up, in the air, within the cubes, between dosings. The fact that the water proceeds downwards by successive stages, moving only during (and a little after) the separate dosing episodes, characterizes this particular type of downward travel of the water as what may be termed champagne-fountain type, or bucket-brigade type, of downward movement.
When a dose of wastewater is applied to a cube of absorbent foam material, that cube becomes saturated, and overflows. When the dosing episode ceases, the excess water drains out of the cube, until only a remnant volume is retained in the cube. The magnitude of this remnant or retained volume depends on such characteristics of the material as its permeability, absorbency, capillarity, etc. It is desirable, for effective wastewater treatment, that this remnant volume be a substantial portion of the cube.
The magnitude of the volume retained in the particular cube depends also on the manner in which water drains out of that cube of absorbent material. Suppose the cube is so arranged in the heap of cubes that the underside of the cube is considerably squashed against the cube beneath: now, water can drain easily out of the upper cube into the lower cube, through what amounts to a hydraulically open connecting bridge or drain connection between the upper and lower cubes.
The more tightly the upper and lower cubes are squashed together, the larger the cross-sectional throat area of the drainage bridge between them, and the easier it is for water to drain out of the upper cube, into the lower cube. It follows that the magnitude of the remnant volume of water retained in the upper cube is inversely proportional to the throat area of the drainage-bridge between the upper and lower cubes—the larger the drainage-bridge throat area between the cubes, the smaller the volume of water retained in the upper cube, between dosings. Or, in other words: the narrower the throat of the drainage-bridge, the more it inhibits drainage.
Therefore, it may be regarded, as a first generality, that the throats of the drainage-bridges between the cubes should be small, in order that the volumes of retained water in the cubes, between dosings, may be large. The greater the remnant volumes of water that are retained in the (many) cubes, between dosings, the greater the aeration effect of the heap of cubes, as a whole, on the water being aerated, during the periods between dosings. Where the drainage-bridge throat between the cubes is very small, it can be expected that the magnitude of the remnant volume of water retained in the upper cube would then be maximised. (Theoretically, the maximum remnant volume of water retained in the upper cube would occur if the upper cube were not touching the lower cube, at all.)
However, there can be a disadvantage to providing too large a remnant or retained volume in the cubes. That is to say: it can be disadvantageous for the throat of the drainage bridge between adjacent cubes to be very small (or zero). The disadvantage, when the drainage bridge is very small, is that, when a new dose is applied to the upper cube, the excess water tends to drain down from the upper cube—not by draining through the (very small) drainage bridge throat between the cubes—but by flowing down the outside surfaces of the cubes.
What would happen, if this effect were to predominate, is that the remnant water inside the upper cube, though large in volume, would become largely isolated, and relatively unaffected by the dosing episode. That is to say: the water that drains down from the upper cube to the lower cube, during the dosing episode, would be the same water that has just been applied to the upper cube; the wastewater actually residing inside and within the upper cube would stay where it is, during the dosing episode, i.e would stay inside the upper cube. Thus, what can happen, if and when the drainage-bridge throat is too small, is that the volume of water retained inside the upper cube, though large, can become stagnant.
In short, the drainage-bridge throat areas between adjacent cubes of the filter medium should not be too large, but neither should they be too small. Where the drainage-bridge is too small (e.g if the cubes are barely touching), the volume of wastewater retained in the upper cube is large, but the remnant water might be stagnant. On the other hand, where the drainage-bridge throat area is too large and wide open (e.g if the cubes are squashed tightly together), then the volume of wastewater retained in the upper cube would be small, between dosings, and in that case, the dose of wastewater might then pass too quickly down through the whole heap of cubes, whereby the heap would be only marginally effective to aerate the wastewater. It follows that a key to effective and thorough treatment of the wastewater is to ensure that the drainage-bridge throat-areas are neither too large nor too small.
It has been recognised that, fortunately, the extremes at which the above-described disadvantageous effects occur do leave a considerable intermediate range. Thus, it is readily possible for the designer to engineer a heap of cubes of absorbent foam material such that, on the average, the throat-areas of the drainage-bridges between the cubes are neither too large nor too small. As disclosed in U.S. Pat. No. 6,153,094, one way in which the designer can arrange for the drainage-bridge throat-areas to be, on the average, the right size was to provide the filter medium in the form of a randomly-arranged heap of five-centimeter cubes of soft plastic foam material. As shown in FIG. 10 of U.S. Pat. No. 6,153,094, the cubes can be contained (loosely) inside a mesh bag.
The technology as disclosed herein is aimed at providing the filter medium in a configuration that provides as effective a degree of treatment as the said random heap of five-cm cubes, but in a configuration in which the filter medium has been provided at a fraction of the cost of the five-cm cubes.
Designers have recognised that, in order to accomplish the best performance, when using the cubes, it is preferred to use a foam that is very permeable, and yet not as soft (i.e physically soft, or squashy) as might be expected in a traditional foam of such high permeability. That is to say: the foam of the cubes should be such that water soaks very easily into the foam, and yet the foam should be mechanically able to resist the weight of the absorbed water, which tends to compress the foam.
It is not difficult to manufacture foam that has these advantageous characteristics. However, the fact that such foam lies a little outside the commercial mainstream of foam manufacture, plus the fact that the foam has to be cut up into regular cubes, means that the five-cm cubes can be rather expensive. The technology as disclosed herein is aimed at providing the filter medium using fewer resources than has been the case in the prior art.